It describes importance of torque V /S speed measurement from designer's point of view
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artificial neural network
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Induction Motor Analysis
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basic procedure for testing induction motor.
electrical engineering.... Induction Motor.... All Topics covered... Add 2 Favorites for future reference... Click "I LIKE THIS" if u like it...Full description
Presentasi tentang Motor Induksi 3 FasaDeskripsi lengkap
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Induction Motor Analysis
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in this project the speed of the induction motor is controlled using power device "TRIAC"Full description
GRAPHS
Measurement of speed control characteristics Speed input potentiometer setting 0 1 2 3 4 5 6 7 8 9 10
f s (Hz)
Nr (rpm)
-
0
1.6667
0.052
2.5333
72.7
4.3333
126.4
6.1833
180.8
7.9833
233.6
10.2167
296.7
13.5500
383
26.3000
762
44.4833
1301
50.9833
1495
Graph of Motor Speed Nr vs. Inverter Inverter frequency fs 1600
Graph of Motor power Ws vs. Inverter output voltage Vs 110
100
90
80
) W ( s W r e w 70 o p r o t o M 60
50
40
30 70
90
110
130
150
Inverter output voltage Vs (V)
170
190
210
DISCUSSION Motive for keeping v/f constant with adjustable speed drives
Necessity of keeping a constant V/f ratio is to maintain a constant torque below the rated speed. Maintaining a constant flux level is needed for a constant torque. Flux depends on the induced emf (E) and the supply frequency, as shown below. E = 4.44 kω Nphf ϕ ϕ α E/f E/f Supply voltage is directly proportionate with the induced emf (E). By maintaining supply voltage to frequency ratio at constant over the speeds below rated, we can have a constant torque output. Also it is important to maintain flux level just below the knee point (90% of the knee point) to ensure proper utilization of the machine. V/f should be such that it won’t exceed the knee point flux level. Otherwise, huge magnetizing currents will draw through the coil damaging the machine. V is not increased with f for speeds above the rated speed, the effect of this on the motor torque would be …
One can think that we can maintain the V/f ratio a constant for all the speeds such that we can have a constant torque output all the time. If that is the requirement you need to maintain the voltage above the rated value for the speeds above rated. Rated voltage of the machine is defined by the insulation of the conductors. This is not a good practice, to maintain maintain voltage above the rated because the insulation won’t be able to withstand the excess voltage. Therefore, above the rated speed voltage is kept constant at rated value while the frequency is increased. This will reduce the air gap flux of the motor, hence the maximum torque output. This region of operation is called the constant power region.
Speed-Torque characteristics of an IM
Degree of speed controllability with frequency control and voltage control
Frequency Control Frequency control of the motor is available for all the speeds, either below or above the rated speed. When going away from the rated frequency the torque is reduced at both high and low frequencies. At lower frequencies, it is a must that V/f ratio should be kept a constant to avoid saturation of magnetic flux. At higher frequencies, voltage should be kept at the rated value to avoid any insulation failures.
Voltage control Voltage control is another option for speed controlling of IM. Motor pullout torque remains dropping as the voltage is decreased. However no load speed of the motor remains a constant in this case and small variation of the speed for a given load can be obtained by voltage control. Controllability is very narrow compared to the frequency controlling phenomena.
Comment on the motor voltage and current waveforms
Input voltage waveform is a 40Hz signal which is generated by six step VSI. It is a square waveform. Current wave form is also in the same frequency and we can observe a clear phase shift with the voltage wave wave form. Spikes can be observed in the current wave form which which are caused by switching of thyristors. Current wave form is highly distorted and that is because the harmonic content is higher in the current wave form. Salient features of control circuit
VR1 & VR2 in slow up/slow down circuit – used – used to change the rate of change of speed signal. There are five different potentiometers in N-I amplifier circuit namely VR1, VR2, VR3, VR4 and VR5. VR1 & VR3 - used to adjust the voltage and current feedback ratios VR2 & VR4 - used to adjust the control signal transient VR5 - used to adjust range of the final signal Since the voltage feed back potentiometer affect the converrter section only it can be used to change Vdc without affecting f s.
